1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly to the improvement of a pneumatic tire which has excellent running performance on snow or icy road surfaces.
2. Description of the Related Art
In order to prevent dust pollution caused by spikes or studs projecting from tread portions, the use of spiked tires has been banned. Since then, various types of tires which do not have spikes and which have excellent running performance on snow or icy road surfaces have been developed. These are generically called studless tires. The characteristics which are substantially common to studless tires, include having the tread divided into a plurality of block-shaped land portions. As shown in FIG. 10, a plurality of vertical and transverse sipings 72, which extend straight in the depth direction of the block-shaped land portion 70, are formed on each of the divided block-shaped land portions 70. The tread land portions are formed block-shaped so that their grip on ice is improved, and the block-shaped land portions are subdivided by the transverse sipings so that the coefficient of friction on ice (edge effect) is improved by the plurality of block square portions which were formed by the subdivision.
However, in order to obtain a higher coefficient xcexc of friction on ice using conventional technology, it is necessary to increase the number of sipings. When the number of sipings is increased, ground-contacting performance deteriorates due to a decrease in the rigidity of the block-shaped land portions and the overall performance is not improved. In addition, irregular wear (heal-and-toe) is generated due to the deterioration of ground-contacting performance.
The present invention was developed in light of the above drawbacks, and an object thereof is to provide a pneumatic tire in which the deterioration of ground-contacting performance while increasing the number of sipings can be prevented, and as a result, wet grip performance, braking performance on ice, traction performance, and irregular wear resistance can be improved.
A pneumatic tire according to a first aspect of the present invention is a pneumatic tire provided with a tread divided into a plurality of block-shaped land portions, the periphery of each of the block-shaped land portions being bordered by a plurality of main grooves extending in the circumferential direction of the pneumatic tire, and by a plurality of lug grooves extending in the transverse direction of the pneumatic tire and intersecting the main grooves, and each of the block-shaped land portions having a plurality of sipings, the siping""s comprising: a first linear portion which essentially extends in a direction perpendicular to the surface of the block-shaped land portion and contacts the surface; a second linear portion which essentially extends in a direction perpendicular to the surface of the block-shaped land portion and is separated from the surface of the block-shaped land portion; and a sloping line portion which connects the first linear portion and the second linear portion and is inclined in a direction towards the wall surface of the block-shaped land portion, wherein the position of the sloping line portion is set to between 10% to 60% of the siping depth.
Accordingly, in the pneumatic tire according to the first aspect of the present invention, since the position of the sloping line portion is set to the relatively shallow position of 10% to 60% of the siping depth, even when the number of sipings is increased, the bending rigidity of the block-shaped land portion is maintained and deformation is prevented. In this way, because the increase in the amount of deformation of the block-shaped land portions when the number of sipings is increased is less and deterioration of ground-contacting performance can be prevented, wet grip performance, braking performance on ice, traction performance, and irregular wear resistance can be improved.
A pneumatic tire according to a second aspect of the present invention is the pneumatic tire according to the first aspect of the present invention, wherein the tread includes block-shaped land portions each having at least four rows of sipings aligned in a transverse direction, and wherein the position of the sloping line portions of the outer sipings on the block-shaped land portions is at a shallower depth than the position of the sloping line portions of the inner sipings.
Therefore, in the pneumatic tire according to the second aspect of the present invention, as the position of the sloping line portion of the outer sipings is set at a shallower depth than the position of the sloping line portion of the inner sipings within the block-shaped land portion, even if the number of sipings is increased, the bending rigidity of the outer portions of the block-shaped land portion is maintained and deformation is further prevented. In this way, the increase in the amount of deformation of the block-shaped land portion when the number of sipings is increased is further reduced and deterioration of the ground-contacting performance can be further prevented. Consequently, wet grip performance, braking performance on ice, traction performance, and irregular wear resistance can be further improved.
A pneumatic tire according to a third aspect of the present invention is the pneumatic tire according to the first or second aspect of the present invention, wherein the sipings are disposed symmetrically with respect to a center line of each of the block-shaped land portions.
Therefore, in the pneumatic tire according to the third aspect of the present invention, in addition to the effects of the pneumatic tire according to the first or second aspect of the present invention, the bending rigidity of the block-shaped land portion is distributed evenly around the center line of the block.
A pneumatic tire according to a fourth aspect of the present invention is the pneumatic tire according to any of the first through third aspects, wherein the distance between the first linear portion and the second linear portion of the siping (i.e., the step) is not more than half the distance between adjacent sipings.
A pneumatic tire provided with a tread divided into a plurality of block-shaped land portions, the periphery of each of the block-shaped land portions being bordered by a plurality of main grooves extending in the circumferential direction of the pneumatic tire, and by a plurality of lug grooves extending in the transverse direction of the pneumatic tire and intersecting the main grooves, and each of the block-shaped land portions having a plurality of sipings, wherein: a wall surface of each of the block-shaped land portions on the lug groove side has a stepped configuration, and the base portion of the block-shaped land portions is larger than the tread portion of the block-shaped land portions.
Therefore, in the pneumatic tire according to the fifth aspect of the present invention, because the wall surface of the block-shaped land portions on the lug groove side has a stepped configuration and the base portion of the block-shaped land portion is larger than the tread portion thereof, even when the number of sipings is increased, the bending rigidity of the block-shaped land portion is maintained and deformation is prevented. In this way, the increase in the amount of deformation of the block-shaped land portion, when the number of sipings is increased, is reduced and deterioration of the ground-contacting performance can be prevented. Accordingly, wet grip performance, braking performance on ice, traction performance, and irregular wear resistance can be improved.
A pneumatic tire according to a sixth aspect of the present invention is the pneumatic tire according to the fifth aspect of the present invention, wherein the sipings have a stepped configuration along the wall surface.
Therefore, in the pneumatic tire according to the sixth aspect of the present invention, in addition to the effects of the pneumatic tire according to the fifth aspect of the present invention, because the sipings have a stepped configuration, the bending rigidity of the block-shaped land portions is reliably maintained and deformation is prevented.
A pneumatic tire according to a seventh aspect of the present invention is the pneumatic tire according to the sixth aspect of the present invention, wherein the sipings are disposed symmetrically with respect to a center line of each of the block-shaped land portions.
Therefore, in the pneumatic tire according to the seventh aspect of the present invention, in addition to the effects of the pneumatic tire according to the sixth aspect of the present invention, the bending rigidity of the block-shaped land portions is distributed evenly around the center line of the block.
A pneumatic tire according to an eighth aspect of the present invention is the pneumatic tire according to the sixth aspect of the present invention, wherein the position of the stepped portion of the wall surface is set closest to the tread surface, and the closer the siping is to the center line of the block-shaped land portion, the deeper the position of the stepped portion of that siping is set, i.e., the closer to the bottom of the lug groove.
Therefore, in the pneumatic tire according to the eighth aspect of the present invention, in addition to the effects of the pneumatic tire according to the sixth aspect of the present invention, the bending rigidity of the block-shaped land portion becomes gradually higher from the center line of the block to the wall surface of the block-shaped land portions at the lug groove side. Thus, the bending rigidity of the outer portions of the block-shaped land portions is maintained and deformation is further prevented.
A pneumatic tire according to a ninth aspect of the present invention is the pneumatic tire according to any of the fifth through eighth aspects, wherein the size of the steps of the wall surfaces is less than 4 mm.
Therefore, in the pneumatic tire according to the ninth aspect of the present invention, in addition to the effects of the pneumatic tire according to any of the fifth through eighth aspects of the present invention, since the volume of the groove portion can be guaranteed by a predetermined amount or more, reduction of wet braking performance can be prevented.
A pneumatic tire according to a tenth aspect of the present invention is the pneumatic tire according to any of the fifth through ninth aspects, wherein the depth of the stepped portions of the wall surfaces is 10% to 60% of at least one of the height of the block-shaped land portion and the siping depth.
Therefore, in the pneumatic tire of the tenth aspect of the present invention, in addition to the effects of the pneumatic tire according to any of the fifth through ninth aspects of the present invention, because the position of the stepped portion of the wall surface of the block-shaped land portions at the lug groove side is set relatively closer to the tread surface, even when the number of sipings is increased, the bending rigidity of the block-shaped land portions is maintained and deformation is prevented.
A pneumatic tire according to an eleventh aspect of the present invention is the pneumatic tire according to any of the fifth through tenth aspects, wherein the size of the steps of the sipings is not more than half the distance between adjacent sipings.
The pneumatic tires according to the first through eleventh aspects of the present invention can be manufactured in accordance with a present tire manufacturing method.